Diode detectors had been known in the art from the first days of radio. They are used to demodulate radio frequency (RF) or any other alternating current (AC) signal and turn it into voltage representative of the modulating waveform. A common diode detector is shown in FIG. 1. AC, sonic, sub sonic, ultra sonic or RF energy and the like (commonly referred to as RF hereinafter) is rectified by diode D1 that acts as a non-linear element for the conversion. While a Schottky diode is preferred, alternative asymmetric junctions are also suitable. The rectified signal is integrated by the capacitor C1 which also serves as the ground return path for the RF signal.
Additional components are frequently included to improve functionality. By way of example, inductor L1 may be employed to provide electrical matching. Resistor R1 and capacitor C2 offer a low pass filter for additional integration time and improved signal to noise ratio. Capacitor C3 is frequently employed to decouple the incoming RF from DC voltages in the circuit.
Those skilled in the art have long recognized that the diode detector may be used to generate voltage that is representative of the AC power inserted into the detector circuit. A typical diode detector circuit as shown in FIG. 1 has a wide frequency response, and a reasonably low conversion loss in the order of six or seven dB.
The circuit however, suffers from poor thermal stability. Several solutions are known in the art to improve stability over a wide temperature range, and are based generally on placing a pair of matching diodes in a the same ambient environment, exposing only one to the RF signal (a detector diode), and utilizing the other (a compensation diode) to cancel or minimize the effects of temperature on the first. An example of such circuit may be seen in FIG. 2, which utilizes a differential amplifier to reduce effects of the thermal characteristics of the diodes. Differential amplifier 25 amplifies the difference between the signal induced in each of the diodes and thus compensate for temperature drift. While the circuit of FIG. 2 offers temperature compensation, the cost, size, weight, and power requirements of the differential amplifier are prohibitive for many applications. The need for positive 24 and negative 22 power supplies further complicates circuit design, and is in stark contrast to the passive nature of the circuit of FIG. 1. Furthermore, the differential amplifiers typically have restricted operating temperature range, whereas the passive components operate over much larger temperature extremes. Thus there are distinct advantages for a detector circuit that does not require a differential amplifier.
Another example of a temperature compensated envelope detector is found in U.S. Pat. No. 4,000,472 to Eastland et al. Eastland teaches a voltage doubler envelope detector, with a forward bias applied to the detector input to shift detector operation out of the nonlinear square law region of operation. The temperature compensation is achieved by having a signal path extending through the envelope detector to one input of a differential amplifier, and having a similar reference path using similar diodes extending to the other input of the same differential amplifier. The difference voltage between the two paths is relatively less affected by temperature variations.
In U.S. Pat. No. 4,820,995 Tamura teaches an envelope detector comprising of passive elements. Essentially, the Tamura system provides a voltage divider, with a series leg having one diode, and an equivalent or similar parallel leg having a second diode. The detector output is from the junction between the legs. As similar DC current flows through both diodes, they exhibit very similar dynamic resistance, and thus one diode compensates for the temperature variability of the other, and the detector has good temperature stability. However, the Tamura circuit suffers a major drawback: Due to the voltage divider, the circuit provides only one half of the voltage provided by a single diode, non-compensated detector. This reduces the signal to noise ratio of the detector by about 6 dB.
In U.S. Pat. No. 6,262,630, Eriksson describes another temperature compensated diode detector. Eriksson provides for a detector diode and a compensation diode in series, and connects an output buffer at the output of the detector diode. As the Eriksson system is also based on a voltage divider, it suffers from a 50% (6 dB) decrease in detection efficiency over the single diode detector, which like the Tamura circuit translates into lower dynamic range and lower signal to noise ratio.
In “A Suppressed Harmonic Power Detector for Dual Band Phones”, (Alan Rixon and Raymond Waugh, APPLIED MICROWAVE & WIRELESS, November 1999, pp. 62–68 (Noble Press)) Rixon et al provide another temperature compensated detector, as seen in FIG. 3. The circuit is designed towards suppressing the second harmonic of the detected signal. When measured with a differential amplifier the Waugh circuit provides good temperature stability but again suffers from the disadvantages of low efficiency and the use of a differential amplifier entails all the liabilities mentioned above.
Voltage multipliers are well known in the art. One may envision the voltage multiplier as a network in which by an arrangement of diodes a plurality of capacitors are connected in parallel to be charged, and connected in series to be discharged, thus effectively doubling the input voltage.
The invention described in U.S. patent application Ser. No. 10/429,151 already incorporated by reference supra, described a temperature compensated diode detector comprising a detector network and a divider network, each having an equal number of diodes therein. The detector network is operative to detect a voltage commensurating with an input signal and to multiply detected voltage by a predetermined factor. The divider network is coupled to the multiplied voltage and is operative to reduce said multiplied voltage. However, at relatively high signal input levels the input resistance at RF becomes a significant and variable load to the RF circuit.
The numerous attempts described above as well as many others point to a clear, and heretofore unfulfilled need in the industry for a high efficiency, temperature stable, envelope detector. The present invention aims to provide such a detector.